The invention relates to an optical disc apparatus and a defocus correcting method for correcting a defocus which is caused due to a chromatic aberration when a recording of data onto an optical disc is started.
In recent years, an optical disc of a high density onto/from which data can be recorded and reproduced has been put into practical use by an optical system obtained by combining a blue-violet laser light source near a wavelength of 400 nm and an objective lens having a numerical aperture (hereinbelow, abbreviated to NA) of 0.85. For example, a Blu-ray Disc (hereinbelow, abbreviated to BD) having a storage capacity of 25 GB or 50 GB has been put into practical use.
As for a light emitting power of the laser light source, although a relatively low power (hereinbelow, called a reproducing power) is used at the time of reproducing data, a relatively high power (hereinbelow, called a recording power) is used at the time of recording data. When the data recording onto the optical disc is started, the light emitting power is switched from the reproducing power to the recording power. However, it has been known that upon switching, a laser wavelength changes instantaneously by about a few nm and what is called a chromatic aberration occurs. When the chromatic aberration occurs, although a defocus occurs, the defocus is soon converged to almost 0 level and becomes a just focus with the elapse of time owing to the tracking operation of a focusing servo. However, since the optical system is not in the just focus state for a period of time until the just focus state is obtained after the start of the recording, recording quality deteriorates.
To solve such a problem, an optical pickup having an optical element for correcting the chromatic aberration has been proposed (for example, refer to the specification of U.S. patent application publication No. 2003/0090801). However, if the chromatic aberration correcting optical element is equipped, such a new problem that the costs of the optical pickup increase occurs. It is, therefore, desirable that the defocus at the time of starting the recording can be corrected in the optical disc apparatus using an optical pickup which does not have the chromatic aberration correcting optical element.
To solve such a problem, for example, in the specification of JP-A-2008-4227, in order to reduce a defocus amount at the moment of starting the recording, a method whereby a focusing error signal (hereinbelow, referred to as an FE signal) and a defocus correction amount which has preliminarily been obtained by learning are added prior to starting the recording has been disclosed.